In a Wireless Local Area Network (WLAN) system, the Distributed Coordination Function (DCF) is the fundamental access method used to support asynchronous data transfer on a best effort basis. The DCF mode of a WLAN system is used to support contention services promoting fair access to the channel for all stations. The multiple access scheme used to achieve these services is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). One way by which stations detect if the channel is busy is by analyzing all detected packets that are sent by other WLAN users and by detecting activity in the channel via relative signal strength from other sources.
Referring to FIG. 1, a wireless communication system 100 includes an Access Point (AP) 105 in communication with a plurality of wireless transmit/receive units (WTRUs), i.e., stations, terminals, 110, 115 and 120. Two WTRUs that are within the coverage area of the AP 105 but outside the coverage area of each other are said to be hidden from each other. If two WTRUs are “hidden” from each other, the first WTRU cannot detect the signals sent by the second WTRU, thus disabling the “collision avoidance” capabilities of both WTRUs in regards to each other.
The WLAN protocol uses a Request-To-Send/Clear-To-Send (RTS/CTS) handshaking mechanism to combat the effects of collisions. By the same account, RTS/CTS can be used to avoid the hidden terminal problem.
Referring to FIG. 2, when RTS/CTS is used, a source WTRU, wishing to transmit a frame, sends an RTS message 205 after the expiration of a Distributed Interframe Space (DIFS) 210 indicating the duration the WTRU needs to transmit its packet. If the destination WTRU successfully receives the RTS message 205, the destination WTRU responds, after the expiration of a Short Interframe Space (SIFS) 215, with a CTS message 220 confirming that the source WTRU is allowed to transmit, and reserving the channel for the transmission of data. The source WTRU then sends a data packet 225, and the destination WTRU then sends an acknowledgement (ACK) 230 to confirm successful reception of the data packet 225. Using this handshake mechanism, all WTRUs are likely to receive at least one of the two messages (i.e., RTS or CTS), since the AP 105 will transmit one of these two. Upon reception of the RTS and/or CTS messages, other WTRUs can set their network allocation vector (NAV) 235, 240, for the duration of the data transmission. A Contention Window (CW) 245 is then established prior to accessing the channel. This mechanism virtually ensures the source WTRU that the medium is reserved for the desired duration, thus solving the hidden terminal problem.
Referring to FIG. 3, the DCF mode of operation also supports fragmentation/reassembly of large media access control (MAC) Protocol Data Units (MPDUs). When the size of an MPDU exceeds a configurable threshold, it is divided into smaller fragments, with the receiver individually acknowledging each fragment. Only the first fragment is sent using the RTS/CTS mechanism. Moreover, the duration fields in the initial RTS/CTS messages only account for the first fragment. Duration information for subsequent fragments is determined by other WTRUs from the header of preceding fragments and acknowledgements (ACKs).
Increased demands for higher range and higher capacity from WLAN systems make the use of adaptive antennas attractive for such systems. Because of the cost associated with adaptive antenna technologies, the use of adaptive antennas is often perceived as being more attractive for the AP than for all WTRUs.
Typically, a smart antenna system uses an antenna array and forms directional beams to transmit and receive radio signals. As this added directivity helps to increase coverage and signal to noise ratio while reducing interference to neighboring Base Station Systems (BSSs), it also impairs the ability for a WTRU in a given beam to perform carrier sensing when the AP 105 transmits packets to WTRUs in other beams. In such a system, using the RTS/CTS handshaking mechanism does not mitigate the hidden terminal problem as WTRUs located in beams, other than the one in which the AP 105 transmits, have a low probability of detecting the RTS (in the case where the AP 105 is the source) or the CTS (when the AP 105 is the destination) sent by the AP 105.
For example, FIG. 1 shows a situation where WTRU 110 and 115, located in a beam 1, send and receive packets to/from the AP 105. Assuming that WTRU 120 is located outside the coverage limits of WTRU 110 and in a different beam (i.e., beam 5) than the beam that WTRUs 110 and 115 are in (i.e., beam 1), it will likely be unable to detect the packets sent by the AP 105 to WTRUs 110 and 115 in beam 1. This is referred to as the hidden beam problem and will result in a collision if WTRU 120 has data to transmit. Also, the fact that WTRU 120 is outside the coverage limits of WTRU 110 translates into a hidden terminal problem, which also results in the possibility of a collision. Thus, using an RTS and CTS handshake will not alleviate the hidden terminal or hidden beam problems because WTRU 120 would be unable to detect neither of the RTS or CTS messages sent by the WTRU 110 to the AP 105, nor those sent by the AP 105 to WTRU 110 or 115 through beam 1.
A method of successfully sending and receiving data packets without experiencing hidden terminal or hidden beam problems is highly desirable.